Cellular and Personal Communication Service (PCS) radio devices must precisely control their radio frequency (RF) power output for the dual purposes of conserving device battery capacity in portable devices when low power transmission is permissible and to maintain transmitted power within acceptable regulated limits at all times. The tasks of monitoring and controlling transmitted power is further complicated since RF devices typically transmit over a broad range of output power levels. Furthermore, the portability of these devices expose them to temperature extremes not normally associated with fixed-base equipment. The combination of exposure to environmental temperature extremes with a broad range of transmission power levels causes similarly broad extremes in device case and device internal temperatures. Prior art devices used to monitor transmission power typically employ semiconductor devices, such as diodes, as signal strength detectors. The forward voltage drop in a forward biased diode is temperature dependent, and therefore temperature compensation is typically employed. One method of providing temperature compensation is to forward bias a reference diode to match the steady-state DC operating characteristics of a detector diode when the device is not transmitting. Using a comparator or differential amplifier, with one input terminal receiving a diode detection signal and a second input terminal receiving a diode reference signal, creates similar input temperature based deviations at both amplifier inputs and therefore compensates for the adverse effects of such temperature variations.
The advent of allocation of a PCS communications bandwidth has motivated many communication system manufacturers and designers to investigate methods of incorporating both PCS band transmission and cellular band transmission into one communications device. Design of component circuits should minimize replicated components in each RF band, thereby reducing the overall complexity and cost of the new dual band devices.